They are not interchangeable. Isometric conditions on storage beyond 200 years and lets projects quantify on a 200-year or 1,000-year basis. Puro.earth issues into a class it describes as at least 200 years. Verra credits on a 100-year horizon with a permanence fraction set by pyrolysis temperature. Rainbow Standard requires at least 100 years and obliges disclosure of which horizon is claimed.
Index
Why the durability number alone misleads
A buyer comparing two suppliers will usually be handed two numbers of years and asked to prefer the larger. That comparison is close to meaningless, because the four registries crediting biochar today do not mean the same thing by a year. One treats durability as an eligibility condition and then quantifies it through a separate storage module. One issues into a named durability class. One applies a discount factor to a fixed hundred-year window, so the same biochar yields fewer credits rather than a shorter claim. One sets a floor and pushes the disclosure obligation onto the project. Same word, four mechanisms, four different quantities of credit from identical material.
The four registries side by side
Where they diverge most
The sharpest divergence is the hydrogen-to-organic-carbon threshold, because it decides whether a given biochar is admissible at all. Isometric's soil storage module sets the tightest bar in the group. Puro.earth and Verra sit at effectively the same, looser level. A biochar measuring around 0.6 therefore qualifies under two of the three and fails the third — not because it is bad material, but because the protocols encode different views of what counts as stable.
The second divergence is structural. Verra's approach ties the credited fraction to pyrolysis temperature, which rewards higher-temperature production directly. Isometric's 1,000-year option instead requires an optical measurement of the biochar rather than a temperature record. These are different theories of how you prove stability: one infers it from how the material was made, the other measures the material itself.
Frequently asked questions
Which registry gives the most credits per tonne?
The wrong question to start with, because the answer depends on the biochar, not the registry. A high-temperature biochar with a low H/Corg ratio does well under a protocol that rewards measured stability. A lower-temperature biochar may be excluded outright by a tighter threshold while still earning a discounted volume under a permanence-fraction approach. The sequence that works is to characterise the material first and choose the pathway second, not the reverse.
Can one project register under more than one registry?
Not for the same tonnes. Registries exist to prevent the same removal being sold twice, so a given batch is credited under one pathway. A producer may in principle operate different sites or different periods under different protocols, but this is a question for each registry's own rules on double issuance, and it should be settled before production rather than after.
Does a longer durability horizon mean a higher price?
Durability is one input into price, not the only one, and the market does not price it linearly. Buyers with science-based targets often have their own internal durability floor, which makes the horizon a gate rather than a premium: below the floor the credit is not purchasable at any price, above it the horizon matters less than volume, delivery certainty and the verification body's reputation.
Conclusion
There is no best registry for biochar, and a supplier claiming otherwise is selling a pathway rather than explaining one. There is a best fit between a specific biochar, characterised in a laboratory, and a protocol whose thresholds that material clears. The useful comparison for a buyer is not which registry credits the longest horizon but whether the supplier can explain why its material belongs in the protocol it chose — and can show the measurement behind the answer.
Sources and methodology
· Isometric, Biochar Production and Storage v1.3 — https://registry.isometric.com/protocol/biochar/1.3 (consulted 28 Sep 2026). The registry publishes no date for this version; none is stated here.· Isometric, Biochar Storage in Soil Environments module v1.3 — https://registry.isometric.com/module/biochar-storage-soil-environments/1.3 (consulted 28 Sep 2026)· Puro.earth, Certification Framework: Biochar, Edition 2025 V2, approved 27 Nov 2025 — https://puro.earth/cdr-infrastructure/methodologies/document-library/ (consulted 28 Sep 2026)· Verra, VM0044 Biochar Utilization in Soil and Non-Soil Applications v1.2, effective 27 Jun 2025 — https://verra.org/methodologies/vm0044-biochar-utilization-in-soil-and-non-soil-applications-v1-2/ (consulted 28 Sep 2026)· Rainbow Standard, Biomass Carbon Removal and Storage (BiCRS) v1.1, 21 Jan 2026 — https://docs.rainbowstandard.io/methodologies/biomass-carbon-removal-and-storage-bicrs (consulted 28 Sep 2026)
Methodology: horizons and thresholds were read from each registry's own methodology document. Where a figure could not be established from the primary document, the cell says so rather than carrying an estimate.
